1. Field of the Invention
The present invention relates to a device housing a battery (or batteries) such as a battery pack or mobile telephone, and to a charging pad that transmits power by magnetic induction to the device housing a battery to charge the battery inside.
2. Description of the Related Art
A charging pad (charging stand, charging cradle) has been developed to charge an internally housed battery by transmitting power from a power supply coil (transmitting coil, primary coil) to an induction coil (receiving coil, secondary coil) by magnetic induction (refer to Japanese Laid-Open Patent Publication H09-63655 [1997]).
JP H09-63655A cites a configuration with a charging pad housing a power supply coil driven by an alternating current (AC) power source, and a battery pack containing an induction coil that magnetically couples with the power supply coil. The battery pack houses circuitry to rectify AC power induced in the induction coil and supply the rectified power to charge the internal battery. With this system, a battery pack can be placed on the charging pad to charge the battery pack battery without direct physical contact.
In the battery charging system described above, AC power induced in the induction coil is rectified by a rectifying circuit and converted to direct current (DC) that can charge the battery. A bridge circuit of rectifying circuit elements such as diodes is used as the rectifying circuit. However, heat generated by the rectifying circuit elements in the bridge circuit is a problem. In particular, when high-voltage rectifying circuit elements are used to prevent damage under certain abnormal conditions, the ON-state voltage drop across the rectifying circuit elements is increased and the amount of heat generated becomes large. In a battery charging system that transmits power by magnetic induction to charge an internally housed battery, it is important to reduce temperature rise in the device housing a battery during charging. This is because heat has a detrimental effect on the battery and internal electronic components.
Rectifying circuit heat generation can be reduced by reducing the voltage drop across the rectifying circuit elements. A synchronous rectifying circuit using field effect transistors (FETs) as the rectifying circuit elements can reduce rectifying circuit element voltage drop and reduce the amount of heat generated. In this synchronous rectifying circuit, FETs connected in the bridge circuit are controlled ON and OFF synchronous with AC power induced in the induction coil to rectify the AC in the same manner as a diode bridge. Since the ON-resistance of the FETs in a synchronous rectifying circuit is low, heat generation can be reduced. This is because FET rectifying circuit element heat generation is proportional to the power loss, which is the FET ON-resistance times the current squared. To reduce the ON-resistance of a semiconductor device such as a FET serving as a rectifying circuit element, the ability of that device to withstand high-voltages (voltage rating) must be reduced. However, if power is supplied from the power supply coil to the induction coil with the output of the rectifying circuit disconnected from the battery, the AC voltage induced in the induction coil will increase. This is because the voltage of the induction coil increases under no-load conditions. If the voltage rating of the rectifying circuit elements is reduced, this situation invites the detrimental effect that induction coil voltage can exceed the voltage rating and damage the rectifying circuit elements. For example, this situation can occur if control of the charging pad malfunctions. Specifically, this situation can occur if the internally housed battery becomes fully charged and the power supply coil remains in an excited state even though a signal to stop charging is sent to the charging pad from the device housing a battery. Further, the no-load condition results from activation of a protection circuit that open-circuits the battery to prevent over-charging.
The present invention was developed with the object of preventing the detrimental effects described above. Thus, it is a primary object of the present invention to provide a device housing a battery and charging pad that can effectively prevent damage resulting from voltages that exceed the voltage rating of the rectifying circuit elements used in the rectifying circuit while reducing heat generation from the rectifying circuit.